In regards to the 2008 Presidential Dollar Sets, the U.S. Mint states:

"The coins in the 2008 United States Mint Presidential $1 Coin Proof Set are proof versions of the general circulating $1 coins scheduled for release, beginning with the James Monroe Presidential $1 Coin in February. These proof coins are struck multiple times with specially polished dies to create a brilliant, sharp relief and mirror-like background. Their frosted, sculpted foregrounds give them a special “cameo” effect. Each coin bears the “S” mint mark representing production at the United States Mint at San Francisco"

Presidential $1 Coin Proof Set price and order information

The set is on sale for $14.95, plus $4.95 for shipping and handling. The Mint indicates there is no order limit on the quantity or number of sets and they can be purchased online through the Mint page: